silabs: add support for EUART peripherals

[loopj]

Is your feature request related to a problem? Please describe.

EUART peripherals are currently not usable as UARTs in Zephyr.

Describe the solution you'd like

Would love to see these added to the uart_silabs_eusart driver, since the underlying "eusart" EMLIB and peripheral HALs support EUART peripherals too.

Describe alternatives you've considered

No response